The mission CHaracterizing ExoPlanets Satellite (CHEOPS) was recently selected by the European Space Agency (ESA). CHEOPS will be the first mission dedicated to observe exoplanet transits by means of ultra-high precision photometry on bright stars. CHEOPS will derive precise planetary masses and radii for an unprecedented sample of near-by planets allowing to set new constraints on the structure, formation and evolution of low mass planets.

The precise derivation of parameters for the discovered exoplanets, including their masses and radius (and thus mean density, critical to get the bulk composition), is strongly dependent on the knowledge of stellar parameters. For instance, the accuracy on the stellar radius is crucial for an accurate determination of the planet radius from a transit light curve. Precisions of a few percent are fundamental to discriminate different planet compositions (Fressin+2011) and allow to fully make use of the high quality data of future missions such as CHEOPS (as well as TESS and PLATO).

The Science: towards precise planetary parameters

It will be addressed, in unprecedented detail, the problem of deriving precise stellar parameters for planet-host stars, in particular focusing on the complex case of M-dwarf stars, whose stellar parameters are difficult to derive with present day methods. New avenues will be explored for the determination of precise stellar parameters that will allow to characterize with unprecedented detail the properties of the exoplanets. It will also be explored synergies between ESPRESSO and CHEOPS (the team is deeply involved in both instruments/missions). ESPRESSO will allow us not only to detect the best targets but also derive the precise planetary mass, and CHEOPS will allow us to better constrain the planetary radius with unprecedented precision. Together they will allow to infer internal properties of the planets with exquisite precision.

Technical contribution for CHEOPS

The technical contribution for the mission science operation relies on the definition of state-of-the-art data reduction strategies. One of the main goals is the definition and evaluation, from the scientific point of view, of the algorithms that will allow the mission to reach the required precision on data. The data reduction package has responsibility co-shared by CAUP and LAM (Laboratoire d'Astrophysique de Marseille).